parent's public document tree
     + parent site images\
     + parent topic 1
     |   + subtopic 1
     |   + subtopic 2
     + parent topic 2
     |
     + child site "root" 1
     |   + child site images
     |   + child topic 1
     |   + child topic 2
     |       + child subtopic 1
     |       + child subtopic 2
     |
     + child site "root" 2
       # and so on